1. Field of the Invention
The invention relates to an optical circuit and other optical elements, and more particularly to a wave guide type optical circuit including an optical wave guide formed on a substrate, and other optical elements.
2. Description of the Related Art
Optical communication systems recently put into practice, have a need for larger capacity and higher performances. In order to accomplish such optical communication, it is necessary to develop optically controllable elements operating at higher speed, such as a light modulator and an optical switch. Such optically controllable elements usually include a diffusion type optical wave guide having a substrate which is composed of electro-optic crystal such as LiNbO.sub.3. (hereinafter, referred to simply as "LN") or LiTaO.sub.3 (hereinafter, referred to simply as "LT"), and into which titanium, magnesium or proton is doped through a surface and thermally doped.
A plurality of wave guide type optical circuits each including the abovementioned diffusion type optical wave guide can be integrated into a single element by means of conventional photolithography, as described by Okayama et al. in "Shingaku Gihou", TSSE 94-214, or by Nishimoto et al. in "Optics" Vol. 1, No. 8, pp.521.
As a light modulator making use of quick response of electro-optic crystal, a Mach-Zehnder type high-speed light modulator has been put into practice. As an optical switch, there has been suggested a digital type optical switch where an output wave guide to which a light is coupled is selected among two or three output wave guides diverged in Y-shape, by concurrently applying voltages having different polarities to the output wave guides.
As another method of forming an optical wave guide constituting an optical circuit, Japanese Unexamined Patent Publications Nos. 2-259608, 7-64034, and 8-5854 have suggested a method of forming a ridge type optical wave guide by means of heteto-epitaxial growth where fused flux is used.
Though not for the purpose of forming an optical circuit, Japanese Unexamined Patent Publications Nos. 3-48832, 4-270322, 5-2201, 5-17295, and 5-100271 have suggested methods of partially inverting crystal orientation of LN or inverting domain in LN. In these methods, LN crystal is used to carry out pseudo-phase-matching in a secondary harmonics generating element.
Japanese Unexamined Patent Publication No. 7-5404 has suggested an electro-optic modulator for compensating for mismatching of a phase speed between an optical signal and a harmonics signal. This electro-optic modulator includes an optical wave guide formed on a ferroelectric substrate and coupled to an optical input terminal. A high frequency wave guide is formed on a substrate in order to apply an electric field to a region located adjacent to the optical wave guide and modulate an optical signal. The ferroelectric substrate includes a ferroelectric domain region for compensating for a difference in a phase within a modulation region. The ferroelectric domain region is periodically inversely rotated and normally rotated.
It is quite important to lower an operation voltage as much as possible in order to accomplish a practical light modulator or a practical optical switch both including LN therein. It is necessary to concurrently apply a voltage of about .+-.50V to two output wave guides in a digital type optical switch. It is advantageous to lower the voltage even to a small degree.
It is necessary in a Mach-Zehnder type optical switch to apply a voltage of about 50V to two phase shifters. Similar to the above-mentioned optical switch, it is advantageous to lower the voltage even to a small degree.
In a high-speed light modulator, a lower operation voltage could accomplish a driver circuit capable of operating at a higher speed. Accordingly, it is desired in a high-speed light modulator to lower an operation voltage as much as possible in order to drive a driver circuit at a higher speed.